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1. Field of Invention
The present invention relates to electronic circuit amplifiers. The invention is more particularly related to increasing the dynamic range of amplifiers while maintaining stability. The invention is useful in many types of circuits, and particularly within an optical transducer.
2. Discussion of Background
Many types of electrical circuits receive input signals and either translate those signals to another format (e.g. optical to electrical, level translations, etc.). A typical optical storage or transmission channel may use some form of optical to electrical system to translate the data modulated light to a corresponding electrical signal such that it may be further processed in a receiver. One type of translation system uses a semiconductor transducer whose current flow is modulated by the light illuminating it and output to a receiver. However, the output current amplitude is too small to be usefully applied directly to the circuits that comprise the data recovery circuits in the receiver and so some form of amplification must be performed. The output current amplitude is dependant on many factors. One significant factor is the light amplitude incident on the transducer. The incident light may be of any arbitrary amplitude, depending upon factors such as transmitter light magnitude, distance between light transmitter and receiving transducer. However, a typical amplifier lacks the required dynamic range for amplifying all the output signals to be applied to the circuits.
U.S. Pat. No. 5,532,471, an embodiment of which is shown in FIG. 1A, illustrates one attempt to solve such amplification requirements. A common emitter gain stage 100 followed by a voltage buffer 110 to drive a shunt feedback network 120 to the input. The feedback network 120 comprises a fixed resistor in parallel with a variable resistor FET device. The effective resistance of the feedback network is controlled by the average amplitude of the output signalxe2x80x94the purpose being to increase the dynamic range.
This implementation achieves a wide dynamic range, but does so at the cost of complexity. To stabilize the amplifier 2 extra FETs are used to track the feedback resistor. One is used to reduce Rc such that the ratio of       p2    p1    =                    R        F            ⁢              C        1                            aC        3            ⁢              R        C            
can be guaranteed to be above a minimum of 2.75. The other FET is used to progressively degenerate the gain stage to further increase the pole frequency ratio by further reducing the open loop amplifier gain, xe2x80x9caxe2x80x9d. The extra circuit complexity is illustrated in FIG. 1B and manifests itself in the requirement for the extra inverting element 118. The increasing emitter degeneration caused by 106 forces the input voltage to increase at higher input amplitudes. This reduces the reverse bias voltage on the transducer as more voltage is dropped across the compound emitter resistance 104,106.
U.S. Pat. No. 5,737,111 uses the same CE gain stage followed by a voltage buffer stage driving a feedback resistor, similar to FIGS. 1A and 1B. However, to accommodate larger input signals, it has some form of limiting diode across the feedback resistor to clamp the signal amplitude. To overcome signal distortion introduced by the clamping mechanism, a DC restore mechanism is introduced by subtracting a DC current from the large amplitude current input signal when an arbitrary amplitude threshold has been crossed.
Meyer et al., IEEE Journal of Solid State Circuits, vol. 29, No. 6, June 1994, Page 701, xe2x80x9cA Wideband Low-Noise Variable-Gain BiCMOS Transimpedance Amplifierxe2x80x9d is a more complex implementation. As shown in FIG. 2, a voltage buffer stage 200 precedes a gain stage 210 and it has a facility to vary the overall gain to accommodate a wide dynamic range. However this is accomplished using 4 FETs as variable resistors to track the main feedback resistor 230.
Specifically, with respect to FIG. 3, in Meyer, the main feedback element is RC. RD tracks RC to control the bandwidth and ensure stability by maintaining separation between the input and output poles. RA is a local shunt to further reduce the gm of the input darlingtonxe2x80x94and hence the loop gain. RB achieves a similar purpose as RA by reducing the stage gm and increasing the large signal handling capability at the input of the amplifier by degenerating the input gm stage.
The final variable resistor, RE, is used to further attenuate the output signal by reducing the differential gain of the output stage.
In Khoman Phang et al., IEEE Transactions on Circuits and Systems II: Analog and Digital Signal Processing, Vol. 46, No. 7, July 1999 xe2x80x9cA CMOS Optical Preamplifier for Wireless Infrared Communications,xe2x80x9d a variable gain approach is used. This approach includes two cascaded common source gain stages connected in a differential configuration.
IEEE Journal of Solid State Circuits, vol. 35, No. 9, Sep. 2000, Page 1260 xe2x80x9cHigh-Gain Transimpedance Amplifier in InP-Based HBT Technology for the Receiver in 40-Gb/s Optical-Fiber TDM Linksxe2x80x9d Jens Mxc3xcllrich et al. shows a similar approach to Phang. With a differential amplifier, with out variable gain control, but an average signal detector. This time the generated average voltage signal is applied as a bias voltage to the opposing amplifier input.
Each of the above solutions provides wider dynamic range for applications such as an optical transducer. However, the circuits have a degree of complexity that reduces the optimum tradeoff between bandwidth and noise as well as increasing cost.
The present inventor has realized the need for low cost wide dynamic range amplifiers that exhibit stability. The present invention provides an amplifier having wide dynamic range and stability by adjusting a gain control resistance of the amplifier such that the pole ratio between the input and output is stable and by using a gain compensation technique to reduce gain stage bias current and, hence the stage gain.
The present invention is embodied as an automatic gain control (AGC) circuit, comprising, a gain stage having, a gain stage amplifier coupled to an input of the AGC circuit, and a gain control mechanism coupled to the gain stage amplifier and configured to vary the gain of the gain stage amplifier; and a gain control circuit coupled to an output of the gain stage amplifier and configured to output a voltage that adjusts the gain of the variable gain control mechanism based on the output of the gain stage amplifier.
The invention may be embodied as an automatic gain control (AGC) circuit, comprising means for varying an amount of gain of the gain stage amplifier based on an output voltage level of the gain stage amplifier; and means for reducing a bias current of the gain stage amplifier based on an output voltage level of the gain stage amplifier.
The invention also includes a method of controlling a gain of a gain stage amplifier over a wide dynamic range of inputs applied to the gain stage amplifier, comprising the steps of varying an amount of gain of the gain stage amplifier based on an output voltage level of the gain stage amplifier, and reducing a bias current of the gain stage amplifier based on an output voltage level of the gain stage amplifier.